In a standard motor-vehicle cooling system sensors are provided for determining the fill level of the liquid. Such sensors can be of the simple float or pressure type and serve mainly to ascertain if the fill level dips below a certain threshold. This information is used to indicate to the driver that the coolant supply should be checked.
Such a system is invariably of the simple on/off type, that is it merely indicates whether or not the fill level is above or below a certain threshold. It gives no indication of how much above or below, that is does not provide an output of relative fill level. In addition the sensors of such systems are notoriously unreliable. Most work poorly in a sometimes violently moving bath of water containing various chemically active substances and have a short service life.
In another known system described in German patent document 41 16 496 a device detects overloading of the coolant pump and alters the timing or fuel feed of the engine so that it runs cooler. Thus this arrangement provides a sort of emergency operation mode, but in no way supplies the user of the engine with information about how much coolant is in the system.